FIG. 1 illustrates a prior art control 10 for controlling the conductivity of a load switch 12 for controlling current flow through an electrical load 14. It should be understood that the control may be used in diverse applications such as driving variable reluctance electric motors. The load switch is a semiconductor switch which may be a bipolar transistor as illustrated. A first switch 16 controls the turning on of the load switch 12 in response to a command from a switch control 18 which turns on the first switch to apply a positive potential from a first DC source 20 to the base of the bipolar transistor 12. Resistor 22 limits the magnitude of the current which flows from the first DC source 20 to the base of the bipolar transistor load switch 12. A second switch 24 controls the turning off of the bipolar transistor load switch 12. The switch control 18 controls the conductivity of the second switch 24 to couple a second DC source 26 which is poled in an opposite direction to the first DC source 20 to reverse bias the base of the bipolar transistor load switch 12. Resistor 28 limits the magnitude of the current flow from the emitter to the base during turning off of the bipolar transistor load switch 12.
The prior art circuit 10 for controlling the conductivity of the load switch 12 has the disadvantage of requiring two DC sources. Two DC sources result in a higher cost and a larger size than the cost and size which would result from a circuit for controlling the conductivity of a load switch with a single DC source.